Iowa Republican Joni Ernst, who won Tuesday's party primary, is slightly ahead of Democratic Rep. Bruce Braley in the race for the seat of retiring Democratic Sen. Tom Harkin, according to the latest Rasmussen Reports poll.

Ernst netted 45 percent of the poll respondents' support, compared to 44 percent for Braley, the poll found. Three percent of the voters said they prefer a different candidate in the race and 9 percent remain undecided.

Braley held a 40 percent to 37 percent lead over Ernst in March. In addition, several other polls held before the primary election showed Braley with a lead of several points over Ernst, Real Clear Politics said, including an 8 point lead in a spring poll conducted by Suffolk University.

Ernst defeated four other Republicans to take the nomination, while Braley was unopposed. Twenty two percent of the voters polled said they regard her "very favorably," and 18 percent said they view her "unfavorably." She has good name recognition, with only 8 percent of those polled saying they had never heard of her.

Braley's very favorable rating was 21 percent, and unfavorable at 18 percent, and 11 percent said they'd never heard of him.

The pollsters surveyed 750 likely voters on June 4 and 5, with a sampling error of 4 percentage points.

Republicans need to take six Senate seats this fall to regain control of the Senate.

Obamacare may influence the Iowa race. Braley, who voted for the law as a House representative, has 96 percent support of the voters who have a very favorable view of Obamacare. But Ernst, whose opposition to Obamacare was a key part of her campaign, earned 85 percent of the choice from respondents with a very unfavorable view of the law.

Braley leads Ernst by 53 percent to 42 percent in voters who consider immigration reform very important, but Ernst leads by 64 percent to 20 percent with voters who say the legislation is not at all important.